ATMOSPHERIC PATH VARIATIONS FOR BASE-LINES UP TO 80 M MEASURED WITH THE SYDNEY-UNIVERSITY STELLAR INTERFEROMETER

Atmospheric turbulence introduces variations in optical path length fo r light from astronomical sources reaching optical instruments on the ground. These have serious implications for an optical/infrared interf erometer, since they will cause variable losses in fringe visibility i f not compensated for within the instrument. Theory predicts that the standard deviation of optical path length variations will increase wit h interferometer baseline to the 5/6 power as long as the turbulence o beys Kolmogorov statistics, but should increase less rapidly for basel ines longer than the outer scale of the turbulence. Estimates of the o uter scale range from a few metres to the order of kilometres. Observa tions made with the Sydney University Stellar Interferometer (SUSI) at baselines of 5, 20 and 80 m are reported which show a dramatic depart ure from the 5/6 power law even for the 5-m baseline, and which sugges t an asymptotic limit to the standard deviation of optical path variat ions of <20 mu m for very long baselines at the SUSI site at the time of the observations.